TETRAARYLBORATES ([AR]4B-) - ESTIMATION OF OXIDATION POTENTIALS AND REORGANIZATION ENERGIES FROM ELECTRON-TRANSFER RATES

Photoinduced electron transfer in ion pairs where the anion is a tetra substituted borate gives a boranyl radical. This species is reactive. When the boranyl radical contains an alkyl group, the carbon-boron bon d cleaves in ca. 250 fs. The lifetime of tetraarylboranyl radicals is greater than 40 ps. The short lifetime of boranyl radicals prohibits m easurement of the oxidation potentials of borates by electrochemical m ethods. These quantities were estimated by application of Marcus elect ron-transfer theory, which relates the energetics to the kinetics of t he reaction. We find that the use of the empirical Rehm-Weller equatio n for this purpose is not appropriate and leads to errors in estimatio n of both the reorganization energy of the reaction and the oxidation potential of the borates. The oxidation potentials were determined in acetonitrile solution by this approach for two series of borates, (tol yl)n(Ph)(4-n)B-, where the tolyl group might be ortho or para substitu ted and n = 0-4. The data reveal that lambda, the reorganization energ y, is 1.0 eV. The oxidation potentials of the borates vary systematica lly in the p-tolyl series from 0.81 to 1.03 V vs SCE with decreasing n umber of tolyl groups on the borate. The o-tolyl series shows higher t han expected oxidation potentials for the tetra- and tritolyl cases, w hich may be due to greater steric hindrance to the acceptor for these two borates.